Coordinate positioning machine

ABSTRACT

A coordinate positioning machine that includes: a structure moveable within a working volume of the machine, a hexapod metrology arrangement for measuring the position of the structure within the working volume, and a non-hexapod drive arrangement for moving the structure around the working volume. Also, a coordinate positioning machine including a structure moveable within a working volume of the machine, a drive arrangement for moving the structure around the working volume in fewer than six degrees of freedom, and a metrology arrangement for measuring the position of the structure within the working volume in more degrees of freedom than the drive arrangement.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to a coordinate positioning machine.Coordinate positioning machines include, for example, coordinatemeasuring machines (CMMs) and machine tools.

2. Description of the Related Art

A non-Cartesian coordinate positioning machine 1 is illustratedschematically in FIG. 1 of the accompanying drawings. The coordinatepositioning machine 1 generally comprises first and second structures 2,4 that are supported and moved relative to each other by a plurality oftelescopic or extendable legs 6 provided between them. The first andsecond structures 2, 4 are sometimes referred to as platforms or stages,and the extendable legs 6 are sometimes referred to as struts or rams.Where there are six such extendable legs 6 (as illustrated in FIG. 1),the machine is commonly called a hexapod.

The extendable legs 6 are typically mounted on the structures 2, 4 viaball joints 8, with each leg 6 either having its own ball joint 8 at oneor both ends thereof (as illustrated in FIG. 1), or sharing a ball joint8 with an adjacent leg 6 at one or both ends. Each extendible leg 6 istypically formed as a pair of tubes, with one tube being movedtelescopically within the other by a drive mechanism (e.g. motor) inorder to provide extension and retraction of the extendible leg 6.

Various relative positions between the first and second structures 2, 4can be achieved by extending the legs 6 by differing amounts, asillustrated in FIG. 1 by arrows 13. The relative position at any instantis monitored by a plurality of length-measuring transducers 10, forexample with one transducer being associated with each extendable leg 6.Each length-measuring transducer 10 may comprise an encoder scale pairedwith a readhead, with the encoder scale being mounted suitably to one ofthe pair of telescopic tubes and the readhead mounted suitably on theother. Extension of the leg 6 thus causes the encoder scale to move pastthe readhead thereby allowing the length of the extendible leg 6 to bemeasured. A computer controller 5 operates to set the length of eachextendible leg 6 to provide the required relative movement between thestructures 2, 4. By having six such length-measuring transducers 10, therelative position can be measured in six corresponding respectivedegrees of freedom (three translational degrees of freedom and threerotational degrees of freedom).

One of the structures 2, 4 is typically provided as part of a fixedstructure of the coordinate positioning machine 1, with the other of thestructures 4, 2 moving 12, 11 relative to the fixed structure. A tool(for example a measurement probe or a drill) can be mounted on themoving structure and a workpiece mounted on the fixed structure, or viceversa, to enable an operation to be performed on the workpiece (forexample measuring, probing, or scanning in the case of a coordinatemeasuring machine, or machining in the case of a machine tool).

For example, as illustrated in FIG. 1, the lower structure 4 is fixedand the upper structure 2 is moveable, with a workpiece 9 mounted on thelower structure 4 and a probe component 3 mounted on the upper structure2. A working volume 14 is defined between the upper structure 2 and thelower structure 4 when at their most spaced-apart positions, with theprobe component 3 being positioned in the working volume 14 by operationof the extendible legs 6. Although arrows 11 are shown to indicatetranslational movement, with appropriate control of the various legs 6the structure 2 could also be tiltable.

Alternatively, the upper structure 2 could be fixed and the lowerstructure 4 moveable, with a probe mounted to a lower surface of thelower structure 4 and a workpiece mounted to a part of the fixedstructure below that, so that the working volume (or operating volume)of the machine is below the lower structure 4 rather than above it.

Various types of non-Cartesian coordinate positioning machine aredescribed in more detail in WO 91/03145, WO 95/14905, WO 95/20747, WO92/17313, WO 03/006837, WO 2004/063579, WO 2007/144603, WO 2007/144573,WO 2007/144585, WO 2007/144602 and WO 2007/144587.

For example, WO 91/03145 describes a hexapod machine tool comprising anupper, moveable, structure that is attached to a base by six hydraulicextendable legs, similar in principle to that illustrated in FIG. 1described above. The extendable legs are attached to the base andmoveable structure via ball joints. The extendable legs are hydraulicand comprise a piston rod that is moveable within a cylinder. The amountof leg extension is measured by mounting a magnetic scale to thecylinder and a suitable readhead on the piston rod. Extension of the legthus causes the scale to move past the readhead thereby allowing thelength of the leg to be measured. A computer controller operates to setthe length of each leg to provide the required movement.

EP3054265A1 discloses the use of a delta robot to move an end effector,and an imaging detector to capture image data of at least part of theend effector. From the captured image data, and in particular fromreference points on the end effector, the position of the end effectorcan be determined by photogrammetry.

DE3504464C1 describes the use of a hexapod arrangement of measurementstruts to calibrate a robot arm. US 2008/0271332 describes the use of ahexapod arrangement of measurement struts in series with a Cartesiancoordinate measuring machine.

SUMMARY OF THE INVENTION

According to an embodiment of one aspect of the present invention, thereis provided a coordinate positioning machine comprising a structuremoveable within a working volume of the machine, a hexapod metrologyarrangement for measuring the position of the structure within theworking volume, and a non-hexapod drive arrangement for moving thestructure around the working volume. The moveable structure is adaptedto carry an operational tool with the metrology and drive arrangementsalso coupled to the moveable structure.

According to an embodiment of another aspect of the present invention,there is provided a coordinate positioning machine comprising astructure moveable within a working volume of the machine, a drivearrangement for moving the structure around the working volume in fewerthan six degrees of freedom, and a metrology arrangement for measuringthe position of the structure within the working volume in more degreesof freedom than the drive arrangement.

According to an embodiment of another aspect of the present invention,there is provided a coordinate positioning machine comprising astructure moveable within a working volume of the machine, a metrologyarrangement for measuring the position of the structure within theworking volume, and a drive arrangement for moving the structure aroundthe working volume, wherein the metrology arrangement comprises aplurality of measurement transducers in a parallel arrangement forproviding a corresponding respective plurality of measurements fromwhich the position of the moveable structure is determinable, whereinthe drive arrangement comprises a plurality of mechanical linkagesarranged in parallel between the moveable structure and a fixedstructure of the machine, and wherein each mechanical linkage isactuated by a drive mechanism which acts between the fixed structure andthe mechanical linkage.

According to an embodiment of another aspect of the present invention,there is provided a coordinate positioning machine comprising astructure moveable within a working volume of the machine, a metrologyarrangement for measuring the position of the structure within theworking volume, and a drive arrangement for moving the structure aroundthe working volume, wherein the metrology arrangement comprises aplurality of measurement transducers in a parallel arrangement forproviding a corresponding respective plurality of measurements fromwhich the position of the moveable structure is determinable, andwherein the drive arrangement comprises a plurality of actuators in aparallel arrangement of a different type to that of the metrologyarrangement. For example, the parallel actuator arrangement may be atri-glide type of arrangement or a cable robot type of arrangement,while the parallel measurement transducer arrangement may be a hexapodtype of arrangement.

The metrology arrangement may comprise a plurality of measurementtransducers in a parallel arrangement for providing a correspondingrespective plurality of measurements from which the position of themoveable structure is determinable.

The measurement transducers may be length-measuring transducers.

The measurements may relate to different respective separations betweenthe moveable structure and a fixed structure of the machine.

The measurement transducers may be adapted to provide directmeasurements of the separations.

The measurement transducers may be adapted to provide directmeasurements of changes in the separations as the structure moves aroundthe working volume, from which changes the separations are determinable.

Each of the measurement transducers may comprise an encoder scale andassociated readhead.

The metrology arrangement may comprise a plurality of extendable legsarranged in parallel, with the extendable legs corresponding in numberto the number of measurement transducers and with each of the pluralityof measurement transducers being associated with a different respectiveone of the plurality of extendable legs.

The coordinate positioning machine may comprise six such measurementtransducers.

The plurality of measurement transducers may be a plurality ofindependent measurement transducers.

The plurality of measurement transducers described herein is to becontrasted with an image-based or photogrammetric metrology arrangement,for example, in which each image capture device does not make a director independent measurement of any length or separation, or of theposition of at least part of the moveable structure; a determination ofthe position of the moveable structure can only take place based on aphotogrammetric combination of images from all of the image capturedevices. With an image-based or photogrammetric metrology arrangement,distances can only be inferred indirectly from image data.

The drive arrangement may be adapted to maintain the moveable structureat a substantially constant orientation as it moves around the workingvolume.

The drive arrangement may comprise a plurality of mechanical linkagesconnected in parallel between the moveable structure and a fixedstructure of the machine.

Each mechanical linkage may be actuated (or driven) by a drive mechanismwhich acts between the fixed structure and the mechanical linkage. Adistinction is to be drawn here between such a drive mechanism and onethat acts for example between two parts (or links) of the mechanicallinkage. For example, with a typical hexapod drive arrangement, themotor for each extendable leg would act between the two parts of theextendable leg, pushing the two parts away from one another to extendthe leg and doing the opposite to retract the leg. The motor does notact between the extendable leg (the mechanical linkage) and the fixedstructure. As such, it can be considered that, in an embodiment of thepresent invention, each mechanical linkage may be actuated (or driven)by a drive mechanism which acts directly between the fixed structure andthe mechanical linkage.

Movement of a driven part of the mechanical linkage may be caused by thedrive mechanism associated with that mechanical linkage rather than byone or more other drive mechanisms associated with other mechanicallinkages of the drive arrangement.

The driven part of the mechanical linkage may be a carriage that isarranged to move linearly along a corresponding track. A plurality ofsuch tracks may be arranged substantially parallel with one another.Three such tracks are provided in the case of a tri-glide drivearrangement.

The drive mechanism may be a rotary drive mechanism, such as is found ina delta robot.

Such a drive mechanism may be arranged to drive a driven part of themechanical linkage in a substantially rotary manner relative to thefixed structure.

The rotary drive mechanism may be a direct rotary drive mechanism.

The drive mechanism may be a linear drive mechanism, such as is found ina tri-glide arrangement or a cable robot.

Such a drive mechanism may be arranged to drive a driven part of themechanical linkage in a substantially linear manner relative to thefixed structure, such as along a substantially linear manner feature ofthe fixed structure, such as along a substantially linear track of thefixed structure.

The linear drive mechanism may be a direct linear drive mechanism.

The linear drive mechanism may be arranged to translate the end of themechanical linkage in a substantially linear manner.

The linear drive mechanism may comprise a linear motor.

Each mechanical linkage may comprise at least two substantially parallelrods to maintain the moveable structure at a substantially constantorientation as it moves around the working volume.

The drive arrangement may comprise three such mechanical linkages.

The drive arrangement may be a tri-glide arrangement.

The metrology arrangement may comprise a plurality of mechanicallinkages arranged in parallel between the fixed structure and themoveable structure, with a corresponding plurality of measurementtransducers associated respectively with the plurality of mechanicallinkages. There may be six such mechanical linkages of the metrologyarrangement and six corresponding respective measurement transducers.

Each mechanical linkage of the metrology arrangement may be connectedbetween points on the fixed structure and the moveable structurerespectively and may be adapted to allow a separation between thosepoints to be varied.

The measurement transducer associated with the mechanical linkage of themetrology arrangement may be adapted to provide an output that isdependent on the separation.

Each mechanical linkage of the metrology arrangement may be anextendable or extending leg.

A mechanical linkage may also be referred to or considered to be akinematic chain or a mechanical assembly.

A drive mechanism may be provided separately for each mechanical linkageof the drive arrangement.

The drive mechanism associated with a mechanical linkage may be arrangedto act between the fixed structure and an end of the mechanical linkage.

Each mechanical linkage may comprise at least one rigid rod.

Each mechanical linkage may comprise at least two substantially parallelrods to maintain the moveable structure at a substantially constantorientation as it moves around the working volume.

Each of the mechanical linkages may be of substantially the samearrangement or design.

A drive arrangement having three mechanical linkages each with a lineardrive mechanism is known as a tri-glide arrangement.

The drive arrangement may comprise or be in the form of or provide adelta robot arrangement.

The drive arrangement may comprise or be in the form of or provide alinear delta robot arrangement.

The drive arrangement may be adapted to move the structure around theworking volume in fewer than six degrees of freedom, and the metrologyarrangement may be adapted to measure the position of the structurewithin the working volume in more degrees of freedom than the drivearrangement.

The metrology arrangement may be a hexapod metrology arrangement.

The drive arrangement may be a non-hexapod drive arrangement.

The drive arrangement may be adapted to move the structure around theworking volume in three degrees of freedom.

The three degrees of freedom may be three translational degrees offreedom.

The metrology arrangement may be adapted to measure the position of thestructure in six degrees of freedom (three translational degrees offreedom and three rotational degrees of freedom, i.e. position andorientation).

The metrology arrangement and drive arrangement may each be arrangedbetween (e.g. connected to) the moveable structure and a fixed structureof the machine.

The metrology arrangement and drive arrangement may each be connected orcoupled to both the moveable structure and the fixed structure.

The drive arrangement may comprise a plurality of actuators in aparallel arrangement. This is to be contrasted with a serial arrangementof actuators that is present, for example, in a traditional three-axis(x, y, z) coordinate measuring machine (CMM).

The drive arrangement may comprise a plurality of actuators in aparallel arrangement of a different type to that of the metrologyarrangement.

The drive arrangement may comprise a plurality of measurementtransducers, separate to those of the metrology arrangement, forproviding corresponding respective measurements from which the positionof the moveable structure is determinable independently of the positiondetermined based on the measurements from the metrology arrangement. Inother words, the drive arrangement may be encoded independently of themetrology arrangement.

The drive arrangement may be coupled to the metrology arrangement via acoupling arrangement which prevents at least some distortion associatedwith the drive arrangement from being transferred to the metrologyarrangement.

The coupling arrangement may be a kinematic or pseudo-kinematic couplingarrangement.

The coupling arrangement may comprise a plurality of balls.

The coupling arrangement may comprise a plurality of resilient spacersor pads.

The moveable structure may comprise a drive part associated with thedrive arrangement and a metrology part associated with the metrologyarrangement, with the drive part of the moveable structure being coupledto the metrology part of the moveable structure via the couplingarrangement.

The drive part of the moveable structure may be coupled to the drivearrangement.

The metrology part of the moveable structure may be coupled to themetrology arrangement.

The coordinate positioning machine may comprise a fixed structure havinga drive part associated with the drive arrangement and a metrology partassociated with the metrology arrangement, with the drive part of thefixed structure being coupled to the metrology part of the fixedstructure via the coupling arrangement.

The drive part of the fixed structure may be coupled to the drivearrangement.

The metrology part of the fixed structure may be coupled to themetrology arrangement.

The moveable structure may carry an operational tool. In other words,the operational tool may be carried by the moveable structure at thesame time as the metrology arrangement is coupled to the moveablestructure. The coordinate measuring machine in such a configuration isready for operational use (with a working tool in place), rather thane.g. merely ready to be calibrated (where there would be no working toolin place).

The machine may be a coordinate measuring machine.

The machine may be a comparator.

The operational tool may be a surface sensing device or measurementprobe.

The machine may be a machine tool.

The operational tool may be a mechanical tool for shaping or machiningmaterials.

The metrology arrangement may comprise measurement transducers arrangedexclusively in parallel with one another, i.e. without any measurementtransducers arranged in series with one another.

The metrology arrangement may comprise six measurement transducers in aparallel arrangement for providing six corresponding respectivemeasurements from which the position of the moveable structure isdeterminable. A metrology arrangement having fewer than six measurementtransducers in a parallel arrangement is not a hexapod metrologyarrangement (e.g. a tripod is not a hexapod).

The metrology arrangement can be considered to be for measuringdifferent positions of the moveable structure within the working volumeresulting from different respective states or configurations of thedrive arrangement. In other words, the metrology arrangement measuresthe position (a first position) of the moveable structure in a firstconfiguration of the drive arrangement, the drive arrangement then movesinto a second configuration different to the first, and the metrologyarrangement measures the position (a second position) of the moveablestructure in the second configuration of the drive arrangement. Thedrive arrangement can be considered to include all parts of the machineused to move the moveable structure from the first position to thesecond position.

The metrology arrangement may comprise six extendable legs arranged inparallel, with the six measurement transducers being associatedrespectively with the six extendable legs.

The measurement transducers may be interferometric measurementtransducers.

The drive arrangement may comprise fewer than six actuators in aparallel arrangement.

The drive arrangement may be a parallel kinematic arrangement.

The drive arrangement may be a non-Cartesian arrangement.

The drive arrangement may comprise fewer than six actuators.

The parallel arrangement of actuators associated with the drivearrangement may be different to the parallel arrangement of measurementtransducers associated with the metrology arrangement.

The drive arrangement may comprise a plurality of measurementtransducers for providing corresponding respective measurements fromwhich the position of the moveable structure is determinable.

The measurement transducers may be mechanical measurement transducers asopposed to optical or image-based or photogrammetric measurementtransducers.

The measurement transducers may be length-measuring transducers.Measuring a length of a part of a machine, such as an extendible leg,may be considered to be equivalent to measuring a separation between twoparts of the machine, such as the ends of the extendable leg. Atransducer may not measure an absolute length or separation, but maymeasure a change in length or separation, from which the absolute lengthor separation can be determined (for example based on a geometric modelof the machine). Examples of sensors that do not measure length, butwhich can be used e.g. in combination with other sensor data todetermine position, are accelerometers (acceleration sensors), tiltsensors and gyroscopes.

The measurement transducers may be sampled at a first clock rate that iscomparable to (e.g. at least half that of, or at least that of, orsubstantially the same as) a second clock rate used to control the drivearrangement.

The measurements received from the measurement transducers may allow theposition of the structure to be determined without reference to othersensor or transducer data (such as photogrammetric data from an imagesensor) that may be obtained at a third clock rate lower than the firstclock rate.

The first clock rate may be higher than 1 kHz, more preferably higherthan 10 kHz, more preferably higher than 15 kHz.

The metrology arrangement may be coupled to the moveable structure in anon-contactless manner, such as optically where the metrologyarrangement is an optical metrology arrangement.

The metrology arrangement may be coupled mechanically to the moveablestructure, such as where the metrology arrangement is a hexapodmetrology arrangement.

The metrology arrangement may be a mechanical metrology arrangement, forexample as opposed to an optical or image-based or photogrammetricmetrology arrangement.

The metrology arrangement may be coupled mechanically to the moveablestructure.

An extendable leg may comprise any mechanical arrangement (e.g.mechanical linkage) that allows the separation between a point on thefixed structure and a point on the moveable structure to be varied.

The moveable structure may be adapted to support or carry an object thatis to be moved around the working volume. The object may be one that isto be picked up and/or placed within the working volume. The object maybe a tool for interacting with or operating on another object, such as aworkpiece, located in the working volume. The tool may be a surfacesensing device. The surface sensing device may be a measurement probe.The measurement probe may be a contact probe. The contact probe maycomprise a stylus which makes physical contact in use with a workpiecesurface to take a measurement. The measurement probe may be anon-contact probe. The non-contact probe may be an optical probe. Thetool may comprise a camera for imaging the surface of a workpiece. Thetool may be a mechanical tool that is typically found in a machine toolfor shaping or machining metal or other rigid materials.

The movable structure may be adapted to carry an operational tool withthe metrology and drive arrangements also coupled to moveable structure.The coordinate positioning machine may be set up with the operationaltool already coupled to the moveable structure and ready to perform theoperation for which it is intended. In other words, the coordinatepositioning machine may be set up for operation rather than merely forcalibration.

The hexapod metrology arrangement may be coupled to the moveablestructure via a different attachment than that used for attaching theoperational tool to the moveable structure.

The hexapod metrology arrangement may be coupled directly to themoveable structure.

A “transducer” can be considered herein to be a device that eitherconverts variations in a physical quantity into an electrical signal (a“sensor”, such as the measurement transducers described herein thatmeasure or sense changes in length), or vice versa from an electricalsignal to a physical quantity (an “actuator”, such as the motors andassociated drive linkages described herein that provide movement to thestructure based on an input or drive signal).

Measuring the “position” of the structure is to be understood asmeasuring the position and/or orientation of the structure, to theappropriate number of degrees of freedom. For example, where position ismeasured in six degrees of freedom then both the position andorientation of the structure are determined. However, if the position isonly measured in three degrees of freedom then this may or may notinclude a determination of the orientation of the structure. The term“measuring the position” is to be interpreted accordingly.

According to an embodiment of another aspect of the present invention,there is provided a method of controlling a coordinate positioningmachine according to the above-described first aspect, the methodcomprising: coupling a tool to the moveable structure, using the drivearrangement to move the tool around the working volume with themetrology arrangement also coupled to the moveable structure, andperforming an operation with the tool.

The method may comprise using the metrology arrangement to determine theposition of the tool within the working volume for the operation.

The method may comprise associating the determined position with theperformed operation.

The operation may be a measurement operation. The operation may be amachining operation.

The tool may be a measurement probe and the operation may be ameasurement operation such as taking a touch trigger measurement of aworkpiece located in the working volume.

The method may be carried out based on the position of the structurerather than the tool, or a combination thereof.

According to an embodiment of another aspect of the present invention,there is provided a controller for a coordinate positioning machine,wherein the controller is configured to perform a method as describedabove.

According to an embodiment of another aspect of the present invention,there is provided a computer program which, when run by a coordinatepositioning machine controller, causes the controller to perform amethod as described above, or which, when loaded into a coordinatepositioning machine controller, causes the coordinate positioningmachine controller to become a coordinate positioning machine controlleras described above. The program may be carried on a carrier medium. Thecarrier medium may be a storage medium. The carrier medium may be atransmission medium.

According to an embodiment of another aspect of the present invention,there is provided a computer-readable medium having stored thereincomputer program instructions for controlling a coordinate positioningmachine controller to perform a method as described above.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1, discussed hereinbefore, is schematic illustration of a hexapodcoordinate positioning machine, having six extendable legs;

FIG. 2 is a schematic side view of the hexapod coordinate positioningmachine of FIG. 1;

FIG. 3 is a schematic side view of a coordinate positioning machineembodying the present invention having a metrology arrangement and aseparate drive arrangement;

FIG. 4 is a schematic side view of a coordinate positioning machineembodying the present invention in which the metrology arrangement isdecoupled to some extent from the drive arrangement;

FIG. 5 shows a first perspective view of a practical embodiment of thecoordinate positioning machine of FIG. 4;

FIG. 6 shows a second perspective view of the embodiment of FIG. 5;

FIG. 7 shows a side view of the embodiment of FIG. 5;

FIG. 8 shows a top view of the embodiment of FIG. 5;

FIGS. 9A to 9E are schematic illustrations of the operation of anembodiment of the present invention;

FIG. 10 is a schematic illustration of a top-down variant of thecoordinate positioning machine of FIG. 4;

FIG. 11 is a schematic illustration of a variant of the top-downcoordinate positioning machine of FIG. 10;

FIG. 12 shows a slight variant of the coordinate positioning machine ofFIGS. 5 to 8;

FIG. 13 shows a top-down variant of the coordinate positioning machineof FIG. 12;

FIG. 14 shows a variant of the top-down coordinate positioning machineof FIG. 13;

FIG. 15 shows another variant of the top-down coordinate positioningmachine of FIG. 13;

FIGS. 16A and 16B schematically illustrate an embodiment having adifferent type of non-hexapod drive arrangement;

FIG. 17 illustrates a practical embodiment of the coordinate positioningmachine of FIGS. 16A and 16B;

FIGS. 18A and 18B schematically illustrate a variant of the embodimentof FIGS. 16A and 16B in which the metrology arrangement is decoupled tosome extent from the drive arrangement;

FIG. 19 schematically illustrates a variant of the embodiment of FIGS.16A and 16B with a bottom-up rather than top-down hexapod metrologyarrangement;

FIG. 20 schematically illustrates a variant of the embodiment of FIGS.16A and 16B in which fixed-length metrology struts are used in thehexapod metrology arrangement;

FIG. 21 schematically illustrates a variant of the embodiment of FIG. 20in which an offset pivot plate is used for the metrology struts;

FIG. 22 schematically illustrates an embodiment having a delta robottype of non-hexapod drive arrangement;

FIG. 23 schematically illustrates a variant of the embodiment of FIG.22, having an increased amount of decoupling between the metrology anddrive arrangements;

FIG. 24 schematically illustrates a variant of the embodiment of FIG.22, having a decreased amount of decoupling between the metrology anddrive arrangements;

FIG. 25 schematically illustrates an embodiment having a serialkinematic type of non-hexapod drive arrangement;

FIG. 26 schematically illustrates another embodiment having a serialkinematic type of non-hexapod drive arrangement;

FIG. 27 illustrates the concept of providing a drive arrangement havingfewer degrees of freedom than the metrology arrangement;

FIG. 28 illustrates a dual hexapod arrangement in which the hexapoddrive arrangement has constrained movement; and

FIG. 29 is a flow diagram representing a method of controlling acoordinate positioning machine embodying the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A side view of the coordinate positioning machine 1 discussed above withreference to FIG. 1 is illustrated schematically in FIG. 2. Thecoordinate positioning machine 1 comprises an upper structure 2 that ismoveable within a working volume 14 of the machine 1. The six extendablelegs 6 form both a hexapod drive arrangement 18 (shown in solid line)for moving the upper structure 2 around the working volume 14, and alsoa hexapod metrology arrangement 16 (shown in dotted line) for measuringthe position of the upper structure 2 within the working volume 14. Thecoordinate positioning machine 1 therefore has combined drive andmetrology struts.

By way of comparison, a coordinate positioning machine 21 according toan embodiment of the present invention is illustrated schematically inFIG. 3. Like the FIG. 2 machine, the coordinate positioning machine 21comprises an upper structure 22 that is moveable within a working volume34 of the machine 21, a drive arrangement 28 (shown in solid line) formoving the upper structure 22 around the working volume 34, and ametrology arrangement 26 (shown in dotted line) for measuring theposition of the moveable upper structure 22 within the working volume34.

Whilst in the coordinate positioning machine 1 illustrated in FIG. 2 thehexapod metrology arrangement 16 and the hexapod drive arrangement 18are combined, in the coordinate positioning machine 21 embodying thepresent invention as shown in FIG. 3, the drive arrangement 28 isdifferent to and separate from the metrology arrangement 26. A technicaladvantage is achieved by separating the metrology arrangement 26 fromthe drive arrangement 28 in this way, because it allows these twodifferent arrangements to be designed with very different (and sometimesconflicting) technical considerations in mind.

Separating and differentiating the metrology arrangement 26 from thedrive arrangement 28 allows the drive arrangement 28 to be maderelatively light-weight and fast, so that the structure 22 can be movedaround the working volume 34 quickly with high accelerations and rapidchanges of direction. Whilst focussing on factors like weight and speedmay sacrifice some degree of positional accuracy in the drivearrangement 28, this is overcome by providing a metrology arrangement 26that is instead designed with positional accuracy in mind.

Because the metrology arrangement 26 is passive and has no need for anydrive components, which add weight and generate heat, metrology errorscaused by inertial and thermal distortion of parts (including themeasurement scale used to measure distance) can thereby be controlledand reduced.

Use of a metrology arrangement 26 that is separate from and different tothe drive arrangement 28 provides a coordinate positioning machine 21 inwhich the moveable structure can be driven quickly around the workingvolume, yet retaining the accuracy required of demanding positioningapplications.

With such a design, it also becomes possible to choose a relativelyinexpensive off-the-shelf drive mechanism for the drive arrangement 28,not designed particularly with high accuracy in mind, knowing that itwill be coupled by a dedicated metrology arrangement 26 to provide therequired accuracy, and this therefore allows production costs to belowered.

Mechanical metrology arrangements also benefit from having low-frictionjoints, while drive arrangements typically require more robust andsubstantial joints that inevitably have a higher degree of friction,particularly when under load, so there is a design conflict that isovercome by separating the metrology arrangement 26 from the drivearrangement 28. The joints of the metrology arrangement 26 can be of alower-friction type and will also not be under the same loads as thedrive arrangement 28. Hysteresis effects, which can lead to differentmeasurements being recorded depending on the direction in which theworkpiece is approached, can accordingly be reduced by separation of themetrology arrangement 26 from the drive arrangement 28.

In the coordinate positioning machine 21 of FIG. 3, the metrologyarrangement 26 is a hexapod arrangement, while the drive arrangement 28is a non-hexapod arrangement (i.e. something other than or different toa hexapod arrangement). Use of a hexapod-based metrology arrangement 26is particularly beneficial because a hexapod provides a robustmechanical system having a parallel arrangement of measurementtransducers that provide direct measurements of distance from which avery accurate and reliable determination of position in six degrees offreedom can be derived.

A hexapod-based metrology arrangement 26 also has advantages over someimage-based metrology arrangements in terms of the rate at which theposition of the moving structure 22 can be determined or sampled. Forimage-based (photogrammetric) metrology arrangements, the sampling rateis inherently limited by the sampling rate of the image sensor, andfurther limited by the time take to perform complex calculations basedon the large amount of image information in order to derive the positionof the moving platform. For example, with the image-based system ofEP3054265A1 it is stated that “the frame rate supplied by the imagingdetectors usually is only up to a few hundred hertz”; this is describedas being a benefit because it avoids detecting unwanted high frequencymovements, like vibrations.

However, the present applicant has appreciated that a far higher dynamicbandwidth can be achieved by direct sampling of data from measurementtransducers associated with the extendable legs of a hexapod metrologyarrangement. For example, a machine controller may typically requestposition data from an absolute encoder every 65 μs (15 kHz), but highersampling rates are also possible. Incremental encoder systems produce acontinuous sinusoidal output which allows finer motion control still,limited only by the rate at which the continuous output can be sampled.Unlike image-based systems, the calculations required to determine theposition of the moveable structure from these values are not overly timeconsuming.

It is also noted that the image-based system of EP3054265A1 does notdescribe the use of a parallel arrangement of measurement transducersthat independently provide a corresponding set of measurements, witheach measurement of the set directly representing or relating to adistance or separation between a different point on the moving platformand a point on the fixed structure, and from which the position and/ororientation of the moving platform may be determined. In particular,EP3054265A1 does not describe the use of a hexapod metrologyarrangement.

With a hexapod drive arrangement such as that illustrated in FIG. 1,each of the six struts requires a motor that must necessarily form partof the associated strut, i.e. that moves with the strut. Therefore, whenthe hexapod is actuated to move the moveable structure around theworking volume, the weight of the relatively heavy motor parts is alsobeing moved around. Having to move this extra mass around reduces thepotential speed (or acceleration) of the drive arrangement, and createsadditional heat in the machine which has a negative effect when itreaches the metrology arrangement. By providing a non-hexapod drivearrangement such as that illustrated in FIG. 3, these problems can beovercome, because it allows the motor parts to be moved off the movingparts.

Furthermore, by using a non-hexapod drive arrangement that providesmovement to the moveable structure 22 in fewer than six degrees offreedom, fewer actuators are required (i.e. fewer than the six actuatorsrequired in a hexapod), reducing cost and complexity and also reducingthe amount of heat generated, due to the fewer number of heat-generatingmotor parts, and therefore improving metrology results.

The hexapod metrology arrangement 26 of FIG. 3 is generally similar tothe hexapod arrangement of FIGS. 1 and 2, but without any actuation ormotor components that are required to provide drive. The drivearrangement 28 in this embodiment is a so-called “tri-glide”arrangement, for example as disclosed in US 2003/0005786, having threecarriages 56 moving along three corresponding respective linear tracks51 arranged substantially in parallel. These arrangements will bedescribed in more detail below with reference to FIGS. 5 to 8.

Referring again to FIG. 3, the coordinate positioning machine 21comprises a lower structure 24 that forms part of the fixed structure ofthe machine 21, with a workpiece 29 mounted on the lower structure 24. Ameasurement probe 30 is supported on the upper structure 22 so that itcan be moved around the working volume 34. The working volume 34 isdefined between the upper structure 22 and the lower structure 24 whenat their most spaced-apart positions, with the probe component 30 beingpositioned in the working volume 34 by operation of the drivearrangement 28.

Also illustrated schematically in FIG. 3 is a controller C forcontrolling the drive arrangement 28 to cause the desired movement ofthe structure 22; the controller C can be implemented in hardware orsoftware or a combination thereof. Purely for the sake of clarity andbrevity, the controller C is omitted from subsequent drawings.

As illustrated in FIG. 4, to provide even further separation between thedrive arrangement 28 and the metrology arrangement 26, the drivearrangement 28 may be coupled to the metrology arrangement 26 via acoupling arrangement 38 which prevents at least some distortionassociated with the drive arrangement 28 from being transferred to themetrology arrangement 26. The coupling arrangement 38 comprises a firstcoupling 38 a associated with the moveable structure 22 and a secondcoupling 38 b associated with the fixed structure 24.

In the schematic embodiment illustrated in FIG. 4, the moveablestructure 22 comprises a metrology part 22 a associated with themetrology arrangement 26 and a drive part 22 b associated with the drivearrangement 28, with the metrology part 22 a of the moveable structure22 being coupled to the drive part 22 b of the moveable structure 22 viathe first coupling 38 a. The metrology part 22 a of the moveablestructure 22 is coupled to the metrology arrangement 26. The drive part22 b of the moveable structure 22 is coupled to the drive arrangement28.

Similarly, the fixed structure 24 comprises a metrology part 24 aassociated with the metrology arrangement 26 and a drive part 24 bassociated with the drive arrangement 28, with the metrology part 24 aof the fixed structure 24 being coupled to the drive part 24 b of thefixed structure 24 via the second coupling 38 b. The metrology part 24 aof the fixed structure 24 is coupled to the metrology arrangement 26.The drive part 24 b of the fixed structure 24 is coupled to the drivearrangement 28.

In this example, each coupling 38 a, 38 b of the coupling arrangement 38is in the form of a kinematic or pseudo-kinematic coupling. In thecontext of locating a body relative to another, kinematic designconsiderations are met by constraining the degrees of freedom of motionof the body using the minimum number of constraints, and in particularinvolves avoiding over constraining. Over constraining can result inmultiple points of contact between two bodies enabling one body to restin more than one position against the other. Accordingly, the body'slocation is not repeatable as it is not known at which of the severalpositions the body will come to rest. In particular, where there is overconstraint, there is a conflict between the constraints that are inplace, so that it is not possible to determine with any certainty whichcombination of constraints will determine the actual position of thebody. These concepts are described in H. J. J. Braddick, “MechanicalDesign of Laboratory Apparatus”, Chapman & Hall, London, 1960, pages11-30.

Such a kinematic coupling, with the minimum number of contact points (orpoint-like contacts) to provide ideal constraint, is also very effectiveat isolating distortions in one half of the coupling being transferredto the other half of the coupling. Thus, the first coupling 38 a helpsto prevent distortions of the drive part 22 b of the moveable structure22 (resulting from forces acting on that part from the drive arrangement28) being transferred to the metrology part 22 a (and thereby to themetrology arrangement 26), and similarly for the second coupling 38 b inrespect of the fixed structure 24. This provides a clearly-delineatedmetrology frame 36 that has a good degree of mechanical isolation fromthe drive arrangement 28.

In particular, in this embodiment each coupling 38 a, 38 b comprises aset of three balls to provide three points of contact according tokinematic design principles (only two are shown in the schematicillustration of FIG. 4). It is also of benefit to use a plurality ofresilient spacers or pads instead of rigid balls, e.g. three suchspacers arranged at the corners of a triangle. This provides some degreeof kinematic coupling, even if the contact is not point-like but insteadspread over the small area of the resilient spacer. Use of resilientspacers (e.g. made of rubber) is beneficial since they act to absorbsome vibration from the drive arrangement 28 so that it is nottransferred to the metrology arrangement 26.

It will also be appreciated that such a coupling can be provided at bothends (i.e. in association with the moveable structure 22 and the fixedstructure 24), or at one end only (i.e. in association with only one ofthe moveable structure 22 and the fixed structure 24), or not at all(i.e. at neither of the moveable structure 22 and the fixed structure24).

An embodiment will now be described in more detail with reference toFIGS. 5 to 8, which show more detailed representations of the machinestructure than the very schematic illustrations of FIGS. 3 and 4.

The hexapod metrology arrangement 26 illustrated in FIGS. 5 to 8comprises six extendable legs 60, generally of the same construction,arranged between the upper structure 22 and the lower structure 24. Asper FIG. 4, the upper structure 22 comprises a metrology part 22 aassociated with the metrology arrangement 26 and a drive part 22 bassociated with the drive arrangement 28, with the metrology part 22 aof the moveable structure 22 being coupled to the drive part 22 b of themoveable structure 22 via the first coupling 38 a. The metrology part 22a of the moveable structure 22 is coupled to the metrology arrangement26 via ball joints 68. The drive part 22 b of the moveable structure 22is coupled to the drive arrangement 28 via ball joints 58.

Similarly, the fixed structure 24 comprises a metrology part 24 aassociated with the metrology arrangement 26 and a drive part 24 bassociated with the drive arrangement 28, with the metrology part 24 aof the fixed structure 24 being coupled to the drive part 24 b of thefixed structure 24 via the second coupling 38 b. The metrology part 24 aof the fixed structure 24 is coupled to the metrology arrangement 26.The drive part 24 b of the fixed structure 24 is coupled to the drivearrangement 28.

Each of the six extendable legs 60 comprises an upper tube 64 and alower tube 62, with the lower tube 62 sliding telescopically within theupper tube 64. The extendable legs 60 are generally of a similarconstruction to those described in WO 2017/021733 and application no.PCT/GB2017/050909, except that there is no need in this embodiment forthe extendable legs to be driven, and therefore no need for anymotor-related components. However, the overall construction of theextendable legs 60 is generally similar.

With the example illustrated in FIGS. 5 to 8, the lower structure 24 isfixed and the upper structure 22 is moveable relative to the lowerstructure 24 by operation of the six extendable legs 60, with ameasurement probe 30 being mounted to a lower surface of the upperstructure 22. In this configuration, a workpiece (not illustrated inFIGS. 5 to 8) would be mounted on top of the metrology part 24 a of thelower structure 24, so that the working volume of the machine 21 isbetween the metrology parts 22 a, 24 a of the upper and lower structures22, 24 respectively. The measurement probe 30 comprises a stylus with aworkpiece-contacting tip, with the measurement probe 30 being connectedto the metrology part 22 a of the moving structure 22 via a quill 32.

The extendable legs 60 are for positioning (i.e. determining theposition of) a component supported by the moveable structure 22 (in theillustrated example the component is the measurement probe 30), or atleast part a specific part of the component (such as the tip of themeasurement probe) within the working volume of the machine.

Upper and lowers ends of each extendable leg 60 are connectedrespectively to the upper structure 22 (specifically, the metrology part22 a of the upper structure 22) and lower structure 24 (specifically,the metrology part 24 a of the lower structure 24) via individual balljoints 68. The upper and lower tubes 62, 64 of each extendable leg 60enclose an elongate member 66, shown in dotted outline in one of theextendable legs of FIG. 5, with an encoder scale 10 affixed to theelongate member 66. The elongate member 66 is itself extendable, forexample by way of a telescopic arrangement. Each elongate member 66extends from its upper joint 68 to its lower joint 68, and it is therespective lengths of the elongate members 66 that determine the precisepositioning and orientation of the metrology part 22 a of the upperstructure 22 (and therefore the measurement probe 30). It is thereforethe length of the elongate members 66 that must be measured preciselyduring a measuring or scanning operation on a workpiece in order todetermine the precise location of the tip of the stylus when it iscontact with the workpiece surface.

The drive arrangement 28 in this embodiment is a so-called “tri-glide”arrangement as described, for example, in US 2003/0005786. The tri-glidearrangement is provided by three mechanical linkages 50 of substantiallythe same design that are connected in parallel between the moveablestructure 22 and the fixed structure 24. Each mechanical linkage 50comprises two substantially parallel rigid rods 52, 54 of fixed length,which act to maintain the moveable structure 22 at a substantiallyconstant orientation as it moves around the working volume 34. Eachmechanical linkage 50 also comprises a carriage 56, with the rods 52, 54being pivotally coupled at their lower end to the carriage 56 and attheir upper end to the drive part 22 b of the moveable structure 22 viaball joints 58.

Three linear tracks 51 are arranged substantially vertically(substantially in parallel) on the drive part 24 b of the fixedstructure 24, with the three carriages 56 being arranged to move along(up and down) the three linear tracks 51 respectively. The three lineartracks 51 effectively form part of the fixed structure of the coordinatepositioning machine 21, and can be considered as an extension to thefixed structure 24 (specifically, the drive part 24 b of the fixedstructure 24). Each carriage 56 is driven in a substantially linearmanner along its corresponding respective track 51 by a linear drivemechanism, with the position of the linear drive mechanism being markedschematically by reference 29 in FIG. 3. The linear drive mechanism maycomprise a linear motor. The linear drive mechanism may comprise astepper motor.

Therefore, each mechanical linkage 50 is actuated by a drive mechanismwhich acts between the fixed structure (linear track) 51 and themechanical linkage 50. More particularly, the drive mechanism actsbetween the fixed structure (linear track) 51 and an end of themechanical linkage 50, i.e. the carriage 56. In other words, the drivemechanism effectively couples the mechanical linkage directly to thefixed structure (“coupled to ground”), providing a force therebetweenwhich acts to push or pull the mechanical linkage (for a linear drivemechanism) or rotate the mechanical linkage (for a rotary drivemechanism). There is no additional moveable linkage between the drivemechanism and the fixed structure, where such an additional linkage cancause movements of the driven part of the mechanical linkage that arenot produced by the drive mechanism itself.

For example, with the “tri-glide” drive arrangement illustrated in FIGS.5 to 8 (and more schematically in FIGS. 3 and 4), each carriage 56 iscaused to be driven linearly up and down its respective track 51 by theaction of the motor (not shown) associated with that carriage 56, andnot by the action of any other motor associated with other carriages 56.In the case of a typical hexapod drive arrangement (as shown in FIG. 1),the motor in a particular extendable leg is only capable of extending orretracting the leg linearly along its length, and yet in use each legwill be moving laterally as well; the lateral movement of a leg (and itsassociated motor) must result from the action of other motors in otherlegs, so that each motor is actually moving the weight of other struts(along with their respective motors).

Therefore, with such a drive arrangement as illustrated in FIGS. 3 to 8,the moving parts can be kept relatively light-weight, in this examplebeing thin and light-weight rods 52, 54, and it is not the case (as itis with a typical hexapod drive arrangement having six extendable legssuch as that shown in FIG. 1) that each motor is moving around theweight of other motors. This allows a light-weight drive arrangementthat is able to move quickly with high accelerations and rapid changesof direction.

Returning to a more schematic format, operation of the tri-glideembodiment will now be described with reference to FIGS. 9A to 9E. Eachof FIGS. 9A to 9E uses a similar representation to that used in FIG. 3,with the two carriages being labelled as 56 a and 56 b respectively andthe two linear tracks being labelled 51 a and 51 b respectively.

Due to the constraints provided by the parallel rods 52, 54 describedabove with reference to FIGS. 5 to 8, motion of the moveable structure22 (by operation of the tri-glide drive arrangement 28) is restricted tothree translational degrees of freedom, so that the moveable structure22 retains a substantially constant orientation as it moves around theworking volume. This constraint to movement in three degrees of freedomis indicated by arrows labelled 3DOF in FIG. 9A.

On the other hand, with six extendable legs 60 of the hexapod metrologyarrangement 26, comprising six corresponding measurement transducers ina parallel arrangement, six corresponding respective measurements areprovided from which the position of the moveable structure isdeterminable in all six degrees of freedom, as indicated by arrowslabelled 6DOF in FIG. 9A.

As illustrated in FIG. 9B, by lowering carriage 56 a and raisingcarriage 56 b along their respective tracks 51 a, 51 b, because the rods52, 54 of the mechanical linkages 50 are of fixed length, the moveablestructure 22 (and along with it the measurement probe 30) is movedleftward and downward within the working volume 34, maintainingsubstantially the same orientation. This causes the extendable leg 60closest to carriage 56 a to shorten and the extendable leg 60 closest tocarriage 56 b to lengthen, with these changes in length being measuredby measurement transducers (e.g. encoders) 10 in the extendable legs 60.From those transducer measurements, the position of the moveablestructure 22 within the working volume 34 can be determined, and becausethe measurement probe 30 is in a known and fixed spatial relationship tothe moveable structure 22, so too can the position of the measurementprobe 30 (and probe tip) be determined. FIG. 9C is the same is FIG. 9B,but without the movement indications for clarity, thereby showing thefinal position of the components after the move operation.

Similarly, as illustrated in FIG. 9D, by raising carriage 56 a andlowering carriage 56 b along their respective tracks 51 a, 51 b, themoveable structure 22 (and along with it the measurement probe 30) iscan be moved rightward within the working volume 34, again maintainingsubstantially the same orientation. This causes the extendable leg 60closest to carriage 56 a to lengthen and the extendable leg 60 closestto carriage 56 b to shorten, with these changes in length being measuredby measurement transducers (e.g. encoders) 10 in the extendable legs 60.From those transducer measurements, the position of the moveablestructure 22 and measurement probe 30 within the working volume 34 canbe determined.

FIG. 9E is the same is FIG. 9D, but without the movement indications forclarity, thereby showing the final position of the components after themove operation.

With the above-described tri-glide embodiment, the extendable legs 60 ofthe hexapod metrology arrangement 26 and the rods 50 of the drivearrangement 28 extend up from the bottom, and that embodiment cantherefore be described as a “bottom up” arrangement. FIG. 10 shows analternative “top down” arrangement, which is generally the same as the“bottom up” arrangement except that the extendable legs 60 of thehexapod metrology arrangement 26 and the rods 50 of the drivearrangement 28 extend down from the top (hence a “top down”arrangement). To enable this, a frame 25 a is provided to support thehexapod metrology arrangement 26 so that it can effectively “hang” fromthe top. The frame 25 a effectively forms part of the fixed structure ofthe coordinate positioning machine 21, as an extension to the fixedstructure 24 (in this case, the hexapod part 24 a of the fixed structure24). As with the previous embodiment, a coupling arrangement 38 a, 38 bis provided to isolate the metrology arrangement 26 from the drivearrangement 28.

Yet another “top down” arrangement is illustrated schematically in FIG.11. This differs from the FIG. 10 embodiment in that the hexapodmetrology arrangement 26 is supported from a frame 25 which extendsaround the top, and is provided inside the tri-glide drive arrangement28. The frame 25 effectively forms part of the fixed structure of thecoordinate positioning machine 21, as an extension to the fixedstructure 24 along with the vertical linear tracks 51 which also becomepart of the frame 25. Furthermore, the FIG. 11 embodiment is notprovided with any coupling arrangement 38 to isolate the metrologyarrangement 26 from the drive arrangement 28.

For comparison with FIGS. 13 to 15, FIG. 12 is provided to show apractical tri-glide embodiment that corresponds closely to thatdescribed above with reference to FIGS. 5 to 8, differing mainly inhaving a closed frame, with extra rigidity and stability being providedto the vertical tracks 51 by way of the top plate of the frame. Like theprevious embodiment, the metrology arrangement of FIG. 12 is decoupledfrom the drive arrangement at least to some extent both at the top (i.e.at the moveable structure) and at the bottom (i.e. at the fixedstructure).

FIG. 13 shows a practical embodiment of the “top down” arrangementillustrated schematically in FIG. 11, but differs from the FIG. 11embodiment by decoupling the drive and metrology to some extent at themoveable structure. FIG. 14 is a variant of FIG. 13, providingdecoupling of the drive and metrology both at the moveable structure andthe fixed structure. FIG. 15 is a further variant, having a separatemetrology frame arranged within a drive frame, with decoupling of thedrive frame from the metrology frame both at the moveable structure andthe fixed structure.

It will be understood that the present invention is not limited toembodiments in which the drive arrangement 28 is in the form of atri-glide. FIG. 16A schematically illustrates an embodiment in which thehexapod metrology arrangement 26 is coupled with a different type ofnon-hexapod drive arrangement 28. Rather than a fixed-length rod 52 oneend of which is driven linearly along a track 51 by a carriage 56 aswith the tri-glide embodiment, in the embodiment of FIG. 16A afixed-length extending rod is instead driven through a pivoting guide 76by a suitable linear drive mechanism provided within the guide 76,thereby changing the separation indicated by the arrow in FIG. 16A andthereby moving the structure 22.

In FIGS. 16A and 16B, similar to FIG. 11, the metrology and drivearrangements 26, 28 are supported in a top-down manner from a frame 25,with the frame 25 forming part of the fixed structure of the coordinatepositioning machine 21. From the position as illustrated in FIG. 16A,when both rods are driven downwards through their respective guides 76,the structure 22 can be moved to the position as illustrated in FIG.16B. As before, the position of the structure 22 is measured by thehexapod metrology arrangement 26.

It will be appreciated that, as with the tri-glide arrangement, eachmechanical linkage of the drive arrangement 28 in the FIG. 16Aembodiment is actuated by a drive mechanism which acts between the fixedstructure and the mechanical linkage, so this embodiment shares the sameadvantage in terms of speed and acceleration.

FIG. 17 illustrates a practical embodiment of theschematically-illustrated arrangement of FIG. 16A. The FIG. 17embodiment is based closely on a non-Cartesian type of coordinatemeasuring machine sold by the present applicant, Renishaw plc, under thetrade mark EQUATOR. The hexapod metrology arrangement 26 is generallysimilar to that of FIG. 5, comprising six extendable legs each having anupper tube 64 and a lower tube 62, with the lower tube 62 slidingtelescopically within the upper tube 64. In this embodiment, theextendable legs are supported in a top-down arrangement from a frame 25to a metrology platform 22 a (part of the moveable structure 22). Thepivoting guides 76 are obscured in FIG. 17 by the structure of the frame25. Three fixed-length drive rods 72 pass through the three pivotingdrive guides 76 respectively and are coupled at their lower end to adrive plate 22 b (part of the moveable structure 22). In this embodimentthe two parts 22 a, 22 b of the moveable structure 22 are separatedspatially by rigid column 23. Three sets of parallel rod pairs 72, 74are arranged to constrain motion in three degrees of freedom, similarlyto the rods 52, 54 of FIG. 5.

Returning to a more schematic representation, FIGS. 18A and 18B show avariant of the machine of FIGS. 16A and 16B, in which the metrologyarrangement 26 is isolated further from the drive arrangement 28. Thisis analogous to the tri-glide embodiment described above with referenceto FIG. 10, so a further description is not necessary. FIG. 19 shows analternative to the FIG. 16A arrangement, with a bottom-up hexapodmetrology arrangement 26 instead of a top-down arrangement.

FIG. 20 schematically illustrates a variant of the embodiment of FIGS.16A and 16B in which fixed-length metrology struts are used in thehexapod metrology arrangement 26, similar to the fixed-length struts ofthe drive arrangement 28 of that embodiment. The six fixed-lengthextending struts as illustrated in FIG. 20 is considered to functionallyequivalent to the six extendable struts of previous embodiments, withthe variable-length part of the strut being indicated by the arrow inFIG. 20; that part is equivalent to the extendable strut of previousembodiments. The term “extendable leg” and “extending leg” are thereforeto be understood herein as being equivalent, meaning any type ofmechanical arrangement or linkage between two points that allows theseparation between those points to be varied. The drive arrangement 28is still a non-hexapod drive arrangement because it only has threeextending struts, as shown in more detail in FIG. 17. FIG. 21schematically illustrates a variant of the embodiment of FIG. 20 inwhich a fixed support (pivot plate) 25 a is used for the metrologystruts that is offset spatially from the fixed support (pivot plate) 25b used for the drive struts.

Embodiments have been described above in which two different types ofnon-hexapod drive arrangement have been employed: a tri-glide lineardrive arrangement (e.g. FIG. 5) and a pivoting linear drive arrangement(e.g. FIG. 17). There are many other possibilities for the drivearrangement, and just a few of these will be described briefly now;others will be apparent to the skilled person.

FIG. 22 schematically illustrates an embodiment having a delta robottype of non-hexapod drive arrangement. A delta robot is a type ofparallel robot, and an example is described in detail in U.S. Pat. No.4,976,582. FIG. 23 schematically illustrates a variant of the embodimentof FIG. 22, having an increased amount of decoupling between themetrology and drive arrangements. FIG. 24 schematically illustrates avariant of the embodiment of FIG. 22, having a decreased amount ofdecoupling between the metrology and drive arrangements. It will beappreciated that, as with the tri-glide arrangement, with a delta robotarrangement each mechanical linkage is actuated by a drive mechanismwhich acts between the fixed structure and the mechanical linkage, sothese delta robot embodiments share the same advantage in terms of speedand acceleration (with a delta robot arrangement the drive mechanism isa rotary drive mechanism, whereas with a tri-glide arrangement the drivemechanism is a linear drive mechanism). Furthermore, with appropriateconstraints (such as described in U.S. Pat. No. 4,976,582) the deltarobot drive arrangement 28 can be adapted to provide movement to thestructure 22 in three degrees of freedom, i.e. in fewer degrees offreedom than is being measured by the hexapod metrology arrangement 26.

The position of the rotary drive mechanism is indicated by reference 27in FIG. 22, while the position of the linear drive mechanism isindicated by reference 29 in FIG. 3. In each case, the drive mechanismacts directly between the fixed structure and the drive arrangement. Inthe case of FIG. 3, the drive mechanism acts to drive the carriage 56(which forms part of the drive arrangement, e.g. as part of a mechanicallinkage 50 as shown in FIG. 5), while in the case of FIG. 22 the drivemechanism acts to drive (rotate) the upper part of the mechanicallinkage that is attached between the moveable structure and the fixedstructure.

Another example of a non-hexapod drive arrangement that is suitable foruse in an embodiment of the present application is a cable-driven robotarrangement (otherwise known as a cable-suspended robot, or just a cablerobot, or a wire-driven robot). This is a type of parallel manipulator(parallel kinematic arrangement) in which a plurality of flexible cablesare used as actuators. One end of each cable is wound around a rotorturned by a corresponding respective motor, and the other end isconnected to the end effector. An example of a cable robot is disclosedin US 2009/0066100 A1. Since cables are typically much lighter than therigid linkages of a serial or parallel robot, the end effector of acable robot can achieve high accelerations and velocities. Because ofthe high measurement rate and dynamic bandwidth achievable with ahexapod metrology arrangement, as well as the high accuracy, thecombination of a hexapod metrology arrangement with a cable drivearrangement is particularly advantageous.

Other types of non-hexapod drive arrangements are also envisaged. Forexample, FIG. 25 schematically illustrates an embodiment having a serialkinematic (as opposed to parallel kinematic) type of non-hexapod drivearrangement, having a plurality of segments or links connected in seriesby rotational joints, with one end of the drive arrangement beingattached to ground and the other end being attached to the metrologyarrangement. As with the embodiment illustrated in FIG. 4, the drivearrangement illustrated in FIG. 25 is attached to the metrologyarrangement via a coupling that helps to prevent drive-relateddistortions being transferred to the metrology arrangement. FIG. 26schematically illustrates another embodiment having a serial kinematictype of non-hexapod drive arrangement, having three parts connected inseries that are moveable respectively along orthogonal axes x, y and z(as marked in FIG. 26). Therefore, the embodiment of FIG. 26 has aCartesian type of serial drive arrangement, whereas the embodiment ofFIG. 25 has a non-Cartesian type of serial drive arrangement. Thesetypes of drive arrangement are well known and no further explanation ofthem is required here.

As explained above particularly with reference to FIG. 9A, the drivearrangement 28 provides three translational degrees of freedom to themoveable structure 22, while the hexapod metrology arrangement 26 isadapted to measure in six degrees of freedom. According to one aspect ofthe present invention, a coordinate positioning machine is proposedwhich comprises a structure moveable within a working volume of themachine, a drive arrangement for moving the structure around the workingvolume in fewer than six degrees of freedom, and a metrology arrangementfor measuring the position of the structure within the working volume inmore degrees of freedom than the drive arrangement. This is illustratedschematically in FIG. 27. One or both of the drive and metrologyarrangements can be a parallel kinematic arrangement, such as a hexapodarrangement, tri-glide arrangement or a delta robot arrangement. Inparticular, it is to be noted that in this aspect the metrologyarrangement need not be a hexapod metrology arrangement.

It is not normal to provide measurement, particularly directmeasurement, in more degrees of freedom than movement. Typically, therewould be N drive parts (rotary or linear) with each drive part beingencoded separately to give N corresponding measurements. For example,for a three-axis CMM there are three driven linear axes, each with aposition encoder, and therefore three corresponding measurements (i.e.driving and measuring both in three degrees of freedom). For a hexapodthere are six variable-length struts, each with a position encoder, andsix corresponding measurements (i.e. driving and measuring both in sixdegrees of freedom).

However, the present applicant has appreciated the desirability andadvantage of being able to provide a drive that is relatively inaccurateand constrained to move in a limited number of degrees of freedom (e.g.three) coupled with a separate metrology arrangement that is highlyaccurate and capable of measuring in all six degrees of freedom, andhence which is capable of compensating for any inaccuracies in themechanically-constrained drive arrangement. For example, where themoving platform is constrained to translate within the working volumewithout rotation, there might be some inadvertent rotation of theplatform caused by distortions or other types of inaccuracy in thestructure, at least some of which may be caused by dynamic effectsassociated with high-speed motion. Such rotations would be detected bymeasuring in more degrees of freedom than driving. It is even possibleto apply the scheme of FIG. 27 to a dual hexapod arrangement asillustrated schematically in FIG. 28, in which the drive hexapod isconstrained to movement in less than six degrees of freedom by anappropriate mechanical constraint.

There are many other forms of non-hexapod drive arrangement, or drivearrangements that are constrained to fewer than six degrees of freedom,as will be apparent to the skilled person. For example, there are manypossible variants of the tri-glide arrangement shown. One variant is toprovide an arrangement having more than three drives and associatedmechanical linkages. And, instead of vertical tracks 51 as illustratedin FIG. 3, the tracks may instead be arranged horizontally, e.g.radially outward from a point, so that movement of the structure 22 isalso effected by movement of the carriages 56 along the horizontaltracks. Many other such possibilities exist.

Although embodiments of the present invention have been described mainlyin relation to the use of a contact probe, in which a stylus of thecontact probe makes physical contact with the workpiece surface to takea measurement, it will be appreciated that the invention is not limitedto contact probes. The same concepts are applicable equally tonon-contact probes, such as optical probes, in which a surface is sensedwithout making physical contact. The invention is generally applicableto any surface sensing device that is adapted to sense a surface,whether by contact or not. The invention can also be applied to thepositioning of a component other than a surface sensing device, forexample for orienting a component part of an article during manufactureof the article. Or, the component could be a tool, or a part thereof,such as a tool typically found in a machine tool for shaping ormachining metal or other rigid materials. The component could be themoveable structure itself. The component may comprise a camera forimaging the surface of the workpiece. The component may comprise an eddycurrent probe for detecting and/or measuring eddy current at or near thesurface of the workpiece. Many other possibilities would be apparent tothe skilled person.

It is to be noted that in an embodiment of the present invention thehexapod metrology arrangement 26 is not provided purely for calibrationpurposes, to be coupled temporarily to the moveable structure to performcalibration of a combined drive and metrology arrangement, and thenremoved for operational use of the machine. Rather, the hexapodmetrology arrangement is intended to remain coupled to the movablestructure to provide position measurements relating to the moveablestructure during operational use. In an embodiment of the presentinvention, in contrast to a calibration-only metrology arrangement, themovable structure is adapted to carry an operational tool with themetrology and drive arrangements also coupled to the moveable structure.The hexapod metrology arrangement may be coupled to the moveablestructure via a different attachment than that used for attaching theoperational tool to the moveable structure. The hexapod metrologyarrangement may be coupled directly to the moveable structure (e.g.rather than via an attachment intended primarily for the operationaltool).

A method of controlling a coordinate positioning machine is illustratedby the flow chart of FIG. 29. In step S1, the metrology arrangement 26is coupled to the moveable structure (or platform) 22. In step S2, thedrive arrangement 28 is coupled to the moveable structure (or platform)22. In step S3, the tool (e.g. measurement probe 30 or cutting tool) iscoupled to the moveable structure (or platform) 22. Thus, at this point,all three are coupled to the moveable structure (or platform) 22. Instep S4, the drive arrangement 28 is used to move the tool around theworking volume 34 (with the metrology arrangement 26 also still coupledto the moveable structure). In step S5, an operation is performed withthe tool, such as performing a touch trigger operation on the workpiecesurface with a measurement probe 30 or performing a machining operationon the workpiece surface with a cutting or machining tool. In step S6,the metrology arrangement 26 is used to determine the position of thetool within the working volume 34 when the operation took place (e.g. toenable the position of the tip of the measurement probe 30 or cuttingtool to be determined). In step S7, the determined position isassociated with the performed operation (e.g. so that a touch triggerevent can be associated with the position measurement for that event).

It will be appreciated that operation of the coordinate measuringmachine 21 can be controlled by a program operating on the machine 21,and in particular by a program operating on a coordinate measuringmachine controller such as the controller C illustrated schematically inFIG. 3. It will be appreciated that control of the extendable legs canbe provided by a program operating on the controller C. Such anoperating program can be stored on a computer-readable medium, or could,for example, be embodied in a signal such as a downloadable data signalprovided from an Internet website. The appended claims are to beunderstood as covering an operating program by itself, or as a record ona carrier, or as a signal, or in any other form.

Although the above embodiments have been described mainly in the contextof a coordinate measuring machine, the concepts are applicable moregenerally to any type of coordinate positioning machine, such ascomparators, scanning machines, machine tools, positioning devices (e.g.for optical components), prototype manufacturing machines and variousother uses.

1-63. (canceled)
 64. A coordinate positioning machine comprising: astructure moveable within a working volume of the machine; a hexapodmetrology arrangement for measuring a position of the structure withinthe working volume; and a non-hexapod drive arrangement for moving thestructure around the working volume, wherein the moveable structurecarries an operational tool with the metrology and drive arrangementsalso coupled to the moveable structure.
 65. The coordinate positioningmachine as claimed in claim 64, wherein the metrology arrangement isadapted to measure the position of the structure in six degrees offreedom.
 66. The coordinate positioning machine as claimed in claim 64,wherein the drive arrangement is adapted to move the structure aroundthe working volume in three degrees of freedom.
 67. The coordinatepositioning machine as claimed in claim 66, wherein the three degrees offreedom are three translational degrees of freedom.
 68. The coordinatepositioning machine as claimed in claim 64, wherein the metrologyarrangement comprises six measurement transducers in a parallelarrangement for providing six corresponding respective measurements fromwhich the position of the moveable structure is determinable.
 69. Thecoordinate positioning machine as claimed in claim 68, wherein themetrology arrangement comprises six extendable legs arranged inparallel, with the six measurement transducers being associatedrespectively with the six extendable legs.
 70. The coordinatepositioning machine as claimed in claim 68, wherein the drivearrangement comprises a plurality of actuators in a parallelarrangement.
 71. The coordinate positioning machine as claimed in claim68, wherein the drive arrangement comprises a plurality of measurementtransducers, separate to those of the metrology arrangement, forproviding corresponding respective measurements from which the positionof the moveable structure is determinable independently of the positiondetermined based on the measurements from the metrology arrangement. 72.The coordinate positioning machine as claimed in claim 64, wherein themetrology arrangement and the drive arrangement are each arrangedbetween the moveable structure and a fixed structure of the machine. 73.The coordinate positioning machine as claimed in claim 72, wherein thedrive arrangement comprises a plurality of mechanical linkages connectedin parallel between the moveable structure and the fixed structure, witheach mechanical linkage being actuated by a drive mechanism that actsbetween the fixed structure and the mechanical linkage.
 74. Thecoordinate positioning machine as claimed in claim 73, wherein the drivemechanism is a linear drive mechanism.
 75. The coordinate positioningmachine as claimed in claim 73, wherein the drive mechanism is a rotarydrive mechanism.
 76. The coordinate positioning machine as claimed inclaim 73, wherein each mechanical linkage comprises at least twosubstantially parallel rods to maintain the moveable structure at asubstantially constant orientation as it moves around the workingvolume.
 77. The coordinate positioning machine as claimed in claim 73,wherein the drive arrangement comprises three of the mechanicallinkages.
 78. The coordinate positioning machine as claimed in claim 64,wherein the drive arrangement is coupled to the metrology arrangementvia a coupling arrangement which is adapted to prevent at least somedistortion associated with the drive arrangement from being transferredto the metrology arrangement.
 79. The coordinate positioning machine asclaimed in claim 78, wherein the coupling arrangement is a kinematic orpseudo-kinematic coupling arrangement.
 80. The coordinate positioningmachine as claimed in claim 78, wherein the moveable structure comprisesa drive part associated with the drive arrangement and a metrology partassociated with the metrology arrangement, with the drive part of themoveable structure being coupled to the metrology part of the moveablestructure via the coupling arrangement.
 81. The coordinate positioningmachine as claimed in claim 78, wherein: the metrology arrangement andthe drive arrangement are each arranged between the moveable structureand a fixed structure of the machine; and the fixed structure comprisesa drive part associated with the drive arrangement and a metrology partassociated with the metrology arrangement, with the drive part of thefixed structure being coupled to the metrology part of the fixedstructure via the coupling arrangement.
 82. The coordinate positioningmachine as claimed in claim 64, wherein the tool is a measurement probe.83. A method of controlling the coordinate positioning machine accordingto claim 64, the method comprising: coupling a tool to the moveablestructure; using the drive arrangement to move the tool around theworking volume with the metrology arrangement also coupled to themoveable structure; performing an operation with the tool; and using themetrology arrangement to determine the position of the tool within theworking volume for the operation.
 84. The method as claimed in claim 83,wherein the operation is a measurement operation.
 85. A non-transitorycomputer-readable medium having stored thereon computer programinstructions for controlling a coordinate positioning machine controllerto perform the method as claimed in claim 83.